docs(tvix/docs/TODO): expand Store Composition config section

Write down some of the thoughts after brainstorming with yuka.

Change-Id: I01c94474dc643b8c4993db80e50d3ec65f5c17f4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11749
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: yuka <yuka@yuka.dev>

tvix/docs/src/TODO.md

@@ -107,9 +107,33 @@ logs etc, but this is something requiring a lot of designing.
 
 ### Store composition
  - Combinators: list-by-priority, first-come-first-serve, cache
- - How do describe hierarchies. URL format too one-dimensional, but we might get
+ - Store composition hierarchies (@yuka).
-   quite far with a similar "substituters" concept that Nix uses, to construct
+   - URL format too one-dimensional.
-   the composed stores.
+   - We want to have nice and simple user-facing substituter config, including
+     sensible default wrappers for caching, retries, fallbacks, as well as
+     granular control for power-users.
+   - Current design idea:
+     - Have a concept similar to rclone config (map with store aliases as
+       keys, allowing to refer to stores by their alias from other parts of
+       the config).
+       It allows both referring to by name, as well as ad-hoc definition:
+       https://rclone.org/docs/#syntax-of-remote-paths
+     - Each store needs to be aware of its "instance name", so it can be
+       included in logs, metrics, …
+     - Have a "instantiation function" traversing such a config data structure,
+       creating store instances and plugging them together, ultimately returning
+       a dyn …Service interface.
+     - No reconfiguration/reconcilation for now
+     - Making URLs the primary data format would get ugly quite easily (hello
+       multiple layers of escaping!), so best to convert the existing URL
+       syntax to our new config format on the fly and then use one codepath
+       to instantiate/assemble. Similarly, something like the "user-facing
+       substituter config" mentioned above could aalso be converted to such a
+       config format under the hood.
+     - Maybe add a ?cache=$other_url parameter support to the URL syntax, to
+       easily wrap a store with a caching frontend, using $other_url as the
+      "near" store URL.
+
 ### Store Config
    There's already serde for some store options (bigtable uses `serde_qs`).
    We might also have common options global over all backends, like chunking